That's... that was a lot to take in. I'm not sure what to add, you already have a lot. I had an idea for a clam that behaves like a man eater but you already have so many unique enemies. That's to be expected of the ocean though huh?
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Sea slime. Personally I would call it a giant amoeba and have it split when it dies like a mother slime, but that's up to you.
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Giant Clam. Looks like a normal clam but up close, like a mimic, it turns aggressive and behaves like a man eater. I thought it could work because you know... clams have tongues and stuff.
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Ocean Pearl
And now that sprites are out of the way, I actually do have something else to add. Critters that you can catch and put in a terrarium or something like krill or an amoeba would be cool. Down in the abyss they wouldn't be able to spawn but higher up they could perhaps? And those neat goggles that you can find could allow you to catch some epic fish unique to the abyss or other darker, deeper parts of the ocean. Also, I remember reading a book once where a child fished up a green violin from a harbor after it floated away from a shipwreck. Perhaps a violin could be put in one of the chests in those shipwrecks? It could be like an instrument from starbound and you you could play songs on it.
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Sadly I don't think that such a large suggestion could make it into the game right now, but you do have my support.
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I'll try to make some sprites after June 22nd, but for now I hope my offerings will do this suggestion good.